In many of today's high technology applications, a complete system can be placed onto a single piece of silicon. The system on a chip (SoC) may include all of the needed electronic circuits and parts for a complete system onto a single integrated circuit (IC). For example, a SoC implementation of a cellular telephone may include a radio frequency transceiver, an analog-to-digital converter (ADC), a digital-to-analog converter (DAC), a microprocessor, memory, audio input/output circuitry, display circuitry, battery management circuitry, peripheral interface circuitry, a clock distribution system, and so forth.
In a 3rd generation (3G) cellular telephone, such as one compliant to the Universal Mobile Telephony System (UMTS) technical standard, the cellular telephone feature the ability to operate in multiple different technical standards and have the ability to connect to other electronic devices via wired and wireless technology. For example, an UMTS cellular telephone also features the ability to communicate using global system for mobile communication (GSM) and wideband code-division multiple access (WCDMA), both of which have different requirements. Furthermore, the UMTS cellular telephone may feature communications interfaces, such as universal serial bus (USB), variants of IEEE 802.11, and Bluetooth, to allow it to connect to computers and other electronic devices. These features are often called connectivity applications.
Each of these applications, since most of them operate at different clock frequencies, will require a different clock in order to operate. Therefore, the SoC implementation of the UMTS cellular telephone may require a clock generator for each of the applications. When combined with time-to-market and cost constraints, many clock generation solutions for a SoC tends towards a operable solution, i.e., a clock generation solution that provides the many different clocks needed.
One easily implemented solution is to use a separate clock generation circuit for each application that needs a different clock. Since in many cases, the designers would already have an existing design for each of the needed clocks, this solution would simply combine all of the needed clocks onto the integrated circuit.
One disadvantage of the prior art is that the use of an unoptimized clock generation circuit can lead to a clock generation circuit that in needlessly complex and may require the use of multiple clock trees that need to be managed concurrently. This can result in a large and cumbersome clock generation circuit that consumes more space on the integrated circuit than needed.
A second disadvantage of the prior art is that the use of an unoptimized clock generation circuit is that the clock generation circuit may not efficiently power-down when the system is powered down. This can result in greater power consumption than necessary and hurting battery life if the system is a wireless device.